The Great A'Tuin

Tuesday, October 4, 2016

HEADS UP! My blog is moving to Wordpress from now on, at thegreatatuin.wordpress.com, because Blogger is proving to be VERY difficult to work with especially when dealing with my inevitable images and graphs. To be determined if I continue in both places or switch over there entirely. I will be putting up notification of this on the home page as well.

So after another entirely too long hiatus, I'm back. This time even more intense work on my PhD ate my time, along with a number of personal things. In whatever case, I didn't exactly have much time to write for myself over the last few months. I'm almost glad though - the last year or so has seen a positive EXPLOSION of amazing origin of life research and I can't wait to pick through it on the record.

But for now before jumping into origin of life research I think I have to instead talk about the Earth itself for a while. So far I've talked about the position of Earth's biosphere in the extremely large context of the history of the universe, our position in time relative to the formation of stars and planets and our position in space relative to different galaxies. This has been informative, but we are necessarily working with very little information and coarse scales here. By comparison within our own solar system there a massive wealth of information that we have nonetheless only barely started to look at. By looking at the history of Earth and comparing it to other objects in our own solar system, I think some very important principles driving the appearance of biospheres like ours become clear and some very important questions we do not know the answers to fall out.

Earth is a very special place. It is a world utterly out of chemical equilibrium.

That is not to say that other worlds in our solar system are in equilibrium - equilibrium is stasis and death, the lack of any energy flow from concentrated sources to diffuse sinks. The atmospheres of Venus and the gas giants act as vast heat engines, transferring the energy of their daylit sides to the nightside and the poles creating winds and clouds in the process. Geology driven by radioactive decay and primordial heat of formation deep inside solid worlds turns over rock through volcanoes and drives chemical cycling of elements into the atmosphere and back into rock. But Earth is a world apart in its disequilibrium. Our atmosphere and upper geosphere are charged up with energy, all this oxygen mixed with methane just itching to oxidize on decadal timescales. The ground itself is full of geologically dredged-up iron and other substances constantly sucking oxygen out of the air on rather longer timescales. And that is to say nothing of all the highly flammable carbon, biomass, ripped out of CO2 and reduced from that highly oxidized form to all kinds of substances and then leftlaying aroundor buried. Anyone with a good enough spectrometer, looking at the Earth across the solar system or even from another star if their instruments were good enough could tell something strange was going on here. A very dynamic, active process transducing lots of energy and utterly remaking the atmosphere and geochemistry for billions of years, creating an extraordinarily reactive atmosphere and oxidizing the crust to great depth.

It's this complete transformation of Earth and only Earth that I think can be explained much more simply than most people think.

Proponents of the 'rare-Earth hypothesis' like to point to everything unique about Earth in our solar system and suggest that unless every single aspect of our world were recapitulated elsewhere, you wouldn't get a big happy biosphere like ours. I'm not a big fan of this approach. Every body in the solar system is the product of a unique history and every body in the universe will be unique if you look close enough. If you look at enough properties, eventually you will convince yourself that the particular combination is unlikely to be found elsewhere and you will probably be right. Be it the large moon birthed in a collision that may have temporarily vaporized a significantfraction of theproto-Earth'smantle, plate tectonics smoothing the carbon cycle and delivering fresh nutrients to the surface constantly, or its particular amount of water, Earth has many unique properties. But with exactly one known biosphere to go on, you run a very real risk ofoverfittingyour models to that one data point. We really cannot say with anycertainty which of these properties might be necessary to create a biosphere in another solar system, or how common such sets of properties could be.

I think we can be more confident when we compare Earth to other objects in our solar system where we have more information. The conditions in this cluster of worlds are diverse, but there is exactly one known biosphere orbiting the Sun. Emphasis on 'known' - I will argue in a moment that our knowledge in this area is absurdly incomplete and there could be half a dozen biospheres in our own solar system that we would never have noticed with the science done so far. Still, ultimately, I would say Earth is unique in exactly one way that matters for any biology and has driven its evolution for at least 3.5 gigayears:

Earth is the only place in the solar system where solvents and clement temperatures meet diverse small-molecule feedstocks and minerals in the presence of large amounts of sunlight.

There's several things going on here. The first few are what most people think of when they talk about the presence of life on a planet: an environment in which something we would call 'life' is chemically possible. The question of what exactly 'life'is isa complicated one and one I'll address when talking about its origin at some point, but for now I'm calling it complicated organic chemistry that can carry heredity and dissipate energy in its environment to do its business.

To allow the kinds of organic chemistry needed by life as we know it, you need a few things in the environment. Firstly,temperature and solvent- it needs to be warm enough for reactions to proceed with some speed and for solvents to be liquid at least some of the time, but not so hot that large complex molecules you need for catalysis and heredity break down into their constituent parts. Life as we know it seems to have a hard limit of ~120 Celsius on the high side, and rather below freezing in the presence of specialized antifreeze chemicals on the low side even if living things can lay dormant doing nothing when they're much colder. Secondly,small-molecule feedstocks. All life on Earth ultimately builds its biomolecules from simple, inorganic molecules - H2O, CO2, and N2 for the vast majority of biomass on Earth. Anything that eats organic molecules ultimately, if you follow the food webs, will get them from something fixing carbon into biomass from CO2, and very nearly all nitrogen in protein ultimately derives from the pool of N2 in the same air. Third,minerals. No living thing on Earth can do without mineral nutrients. Our genetic material itself contains huge amounts of phosphate, an inorganic ion leached from rocks, and almost every functional energy metabolism on the planet depends completely on electrons hopping along beautifully orchestrated chains of iron, sulfur, copper, and molybdenum ions clutched by proteins that are really just acting as scaffolds for them rather than enzymes. Even if we didn't need all that calcium in our bones, minerals are needed as catalysts and components of basic biomolecules.

Last comes what I think is far too often overlooked: energy. All this complex chemistry doesn't just happen. A reaction that is thermodynamically favorable - releasing energy in the form of heat or introducing entropy by moving something from high concentration to low - will happen spontaneously, but before too long everything that can happen hashappened and everything stops. To drive complex chemistry and the production of biomolecules, there has to be energy flux into the system, adisequilibriumto be tapped. And this is where Earth shines. Other places in our solar system have liquid solvents, small molecule feedstocks, minerals, or all three. But there is nowhere else in the solar system that has all those material requirements, PLUS the flux of over a kilowatt of energy per square meter the sun pours down ontothe same surface.

The surface of Mars and thecloudtopsof venus get plenty of sunlight, but Mars is nearlyvacuum-dessicated and UV-radiation-blasted while Venus's clouds are lacking in anything resembling a mineral surface within 70 kilometers even though sulfuric acid might be a workable biological solvent for something that evolved in it. The icy moons of the outer solar system have plenty of liquid water underground at clement temperatures full of dissolved minerals and feedstocks, but these clement environments are all under up to kilometers of ice. Even on Titan, where you might imagine some kind of very low-temperature reactions happening in hydrocarbon solvents, the sunlight at the top of the clouds is something like 1% as bright as here and under the cloud deck it's more like 0.1%.

But on Earth, you have solvent falling from a sky made out of every small molecule feedstock you could want onto mineral surfaces bathed in a kilowatt per square meter of energy. And photosynthesis capturing this vast energy flux is ultimately what makes the Earth's biosphere what it is, splitting water into oxygen and building biomass by combining the resultant hydrogen with CO2 and N2. It is what has transformed the planet so completely you could tell it was alive from light-years away if you could get a quick glimpse of its atmosphere.

I promised when I started this blog to talk about actual observables, what we can know and what we can't with the information actually at hand. In that spirit, after talking about how Earth is so special when it comes to biology in our solar system I need to insert a vital caveat.

There could be half a dozen active biospheres in this solar system right now and we would never know it with the information at hand.

A distinction has to be made between the presence of life, and the presence of a huge high-energy high-biomass biosphere like that of the Earth that chemically transforms aworld. There are plenty of places on Earth that have every attribute I talked about above except sunlight. Fifty years ago you might have assumed that without sunlight to drive the primary producers of biomass there would be no life there - and as we have seen recently, you would have been wrong.

Sunlight is a vast, concentrated energy source as biological energy sources go: 170,000terawattshitting the Earth at all times is NOT shabby, and even if photosynthesis often only captures a small fraction of that it drives the vast majority of the metabolism on Earth. But in recent decades, the discovery of organisms creating biomass from CO2 using energy not derived from the sun has shown that photosynthesis is not the only biological energy source on Earth. Microbes can be found kilometers under the surface of the continents and the ocean, and a large fraction of the biomass of the planet may be living at very low cell densities and metabolic rates kilometers underground where they have no interaction with sunlight or energy derived from it at all. To quote Jan Amend, a geochemist at the University of Southern California, "We keep digging and digging and digging deeper and have not hit the bottom of the biosphere."

Photosynthesis and related activities (i.e. us) on the surface of the Earth definitely represents the majority of the metabolic activity of the planet. But it is NOT all there is, and perhaps more importantly, given how complicated a process photosynthesis in all its forms is it can not have been how the first life on Earth got its energy. Something else has to have been the energy source for the earliest life.

Chemolithotrophs, organisms that fix CO2 into biomass like plants but using energy from small chemicaldisequilibriawithin rock to do it, have been found kilometers deep under the continents. Methanogens andacetogens, organisms that takeH2and oxidize it with CO2 into methane and acetate for their energy (and may have a key role to play in the origin of life on Earth - more on that in a future post) are found anywhere biological processes create H2 but also deep on the bottom of the ocean, living off geological processes that make H2. The total wattage available to these organisms is constrained. I am unsure how to quantify it precisely, but given sheerthermodynamicsit probably has to be comfortably less than the estimated geological heat flux of the Earth (about 47terawatts, ~0.03%of what the planet gets from the Sun) plus a little extra for chemical reactions that occur at the mouths of certain types of hydrothermal vents, but the discoveries in this new sector of the biosphere just keep coming.

Pictured above isDesulforudisaudaxviator, image taken from the Microbe Wiki (https://microbewiki.kenyon.edu/index.php/Desulforudis_audaxviator). This bacterium was discovered kilometers underground in an anoxic aquifer at a pH of 9.3 and a temperature of 60 C. It dies from the tiniest whiff of oxygen, and carries out all the reactions necessary to take rock and N2 and CO2 and turn it into biomass, functions that can be spread out across a dozen species in a surface ecosystem. It is believed to live off the energy ofradioactive decay in the rocks around it that split water and create the tiniest of chemicaldisequlibriafor it to insert itself into as a middleman in their dissipation.

This planet has been crawling with scientists for a few hundred years, and swarming with pretty-damn-smart people for two hundred thousand years before that. D.audaxviatorwas discovered in 2005. These non-solar-driven living things are very easy to miss. They can be detected with the latest molecular biology techniques because we know exactly what we are looking for, chemicals that are shared by all terrestrial living thingsthat we have learned to detect at the single-molecule level, and because of recent advances in telling apart geochemistry from biochemistry which is not always as straightforward as it appears.

What have we done on other worlds in our own solar system compared to the massive, systematic exploration that was needed to find these things on Earth? It'svery hardto do science somewhere by dropping a few kilograms of automated scientific instruments from the sky every few years that cost well over their weight in gold and have severe bandwidth constraints. Bigthings are being missed all the time. It took until 2008 to figure out a basic, extremely important fact about the Martian soil - the fact that it contains up to 0.5% extremely reactive (and toxic)perchloratecreated by the interaction of radiation and ancient salt deposits. Every result from every instrument that probed and sampled the Martian soil from the Viking landers in the 1970s onwards had to be reinterpreted in the light of this finding and it was only conclusively discovered in the first place because one of the Phoenix lander's instruments basically malfunctioned in the presence of the unexpected substance. The geysers ofEnceladus, discovered in 2004 when the Cassini probe reached Saturn, spew the ocean of that tiny geologically active world into space where it can be sampled. The instruments on Cassini are not sensitive enough to resolve the composition in all but the most general of terms - I have been to a talk in which one of the head scientists of the mission shrugged while pointing at a graph from the mass spec and said "there are molecules with at least three carbons in there somewhere."

Would we ever detect the presence of D.audaxviatordeep underground at Mars, or microbes living in hydrothermal vents within Europa orEnceladusfrom this kind of data? At this point, I really don't think so. By my count, there are at least 6 places in our solar system with everything necessary for a low-biomass biosphere and the origin of life (again more on this later) that we simply couldn't detect with the data at hand - underground at Mars, the oceans of Europa, the oceans of Ganymede, the deep underground water oceans of Titan, the surface hydrocarbon lakes of Titan, and the oceans of Enceledus. Count me in halfway for more speculative ideas about thecloudtopsof Venus (where the limiting factor would be availability of minerals and not getting plunged down into the sterilizing depths of the atmosphere) and potential oceans within other icy moons and Kuiperbelt objects. We don't know if there are biospheres in these places, and with current data we can't know. The answer is just insufficiently constrained.

We have onlyjustgotten to the good part of solar system exploration. It wasn't that many decades ago that the planets were only small discs through telescopes to us with rudimentary maps if any. Now they are worlds, and we finally know enough about them and about Earth itself to actually start asking the really interesting questions and to start looking for things that are hard to find. The presence or the absence of another biosphere in our solar system would be extremelyinformative. A second origin of life in our solar system would suggest that biospheres are extremely common, even if you need special circumstances for it to explode in scale and remake a world. Only one originin this system would be a little data in favor of the origin of life being tougher, but we need a LOT of information before wewould be able to say that. Proving a negative is difficult to say the least, and proving that there was never a biosphere elsewhere in our solar system that died out later is even harder.

Apparently every time I write for this blog, I wind up producing huge essays. Up next, I want to talk about the history of life on Earth andits main developmentsover time and what they may (or may not) tell us about the types of biospheres that could exist elsewhere.

Wednesday, March 9, 2016

Well
that was a long hiatus. Between pushing out a paper, TA-ing
two classes, and getting distracted with fiction writing, this blog
went way on the back burner for a couple of months.

I
was frankly surprised and flabbergasted by the degree of interest my
first post got. I just play an astrobiology type on the internet, I'm
really just a molecular biologist with a hobby. I got a number of
interesting pointers to recent research, a lot of interesting
questions, and spoke to a number of very interesting people actually
working in the field of astrobiology! As a result, I'm putting the
next topic (our own solar system and its history) on hold for a
little bit and going a little more in depth into our solar system's
cosmological position in space and time. The more I look into this,
the more clear it becomes to me that we are utterly typical in terms
of where and when you'd expect a complex biosphere to form – on a
coarse scale, at least. Some interesting questions still arise about
the sort of star we find ourselves around.

Anyway,
in my last post I mentioned a recent study (Sobral 2012,
http://arxiv.org/abs/1202.3436)
which looked at star formation rates across cosmological time using
proxy measurements, finding a precipitous decline in star formation
rates for many gigayears. I can do better than this – I think I can
put some rough numbers on our star, planet, and biosphere's relative
position in time and how typical it is drawing from multiple sources.

Numerous
publications for decades at this point have performed similar
measurements of star formation rate over time, and all those with
sufficient detail reveal one, simple, important fact: the rate of
star formation per unit mass in the universe as a whole is decaying
exponentially, and has been for most of the history of the universe.
I could point to a number of publications for this, but for the
purposes of this post I will use Yuskel, 2008:
http://arxiv.org/abs/0804.4008.
This group collated a number of datasets and fit an equation to the
star formation rate going all the way back to when the universe was
less than a billion years old. I've reproduced a figure from Horuchi,
2010 (http://arxiv.org/abs/1006.5751)
which while not actually being ABOUT star formation rates takes the
fit from Yuskel and compares it to multiple data sets, supporting its
validity:

There's
a couple of things going on here. Firstly, note that the star
formation rate is given in terms of something called Z – the
redshift. Astronomers love talking about their observations in terms
of redshift because it is directly observable and related to
both distance and age of what you are looking at. You look and you
see something that should be radiating at wavelength X instead
radiating at wavelength X*(Z+1). That observation is generally
incontestable. But to turn that observation into a distance or time
you need to start messing around with cosmological models which,
although they have been converging to a pretty tight focus in recent
years are still open to refinement and change. It's generally better
to keep your data in the raw observational form as a result.

Secondly,
note the FORM of the equation of best fit. Since around Z = 1,
corresponding to more than eight billion years ago, the star
formation rate has been cratering. As the universe has expanded over
that timeframe, star formation rate has very closely tracked [size of
the universe]^-3.4th. The plateau of steady high star
formation rates lasted only about 3 or 4 gigayears, long before our
Sun formed. This represents huge bursts of star formation from young
galaxies and giant ellipticals that formed and burned out young.

If
I want to get an idea of the total number of stars that have been
born and will ever be born, I need to turn this function of star
formation rate at a given Z (expressed in solar masses formed per
comoving cubic megaparsec per year so as to normalize for the
expansion of the universe) into a function of star formation rate at
a given absolute time,project
it forward with reasonable assumptions, and integrate. Here is a
graph of Z with respect to T from the best modern cosmological
models:

Redshift
changes with time very differently when the universe is
matter-dominated (blue line), has a roughly even amount of matter and
dark energy (red line), and when it is dark-energy-dominated (green
line). Whatever the case, I can use the relationship between redshift
and time to turn the graph of star formation rate across redshift
into star formation rate over time for the history of the universe so
far:

The
red dot represents the formation of the Sun, and the end of the blue
line the present day at t = 13.82 gigayears. As you can see, the vast
vast majority of star formation occurred before our Sun was born.

How
to project this plot forward into the future in order to determine
how many stars will ever live? I have made two executive decisions.
Firstly, I have decided to simply project the exponential decay that
has been occurring since Z=1 forward into the future, for reasons
that I will get to later. Secondly, I have decided to break with a
slavish devotion to the redshift numbers of the far future, once the
universe has turned completely dark-energy dominated. Instead, I will
keep the same function of Z with T and therefore the same function of
star formation over time that has held since approximately Z=1,
because I fail to see how simply increasing the distance between
galaxy clusters more and more rapidly as will happen once the
universe is dark energy dominated will change star formation rates
within the clusters. It doesn't make a huge difference, going one way
or the other changes the final numbers by less than five percent.

Anyways,
after much obsessive faffing around with these equations in matlab to
deal with the above set of assumptions, I projected star formation
rates forward into the future to get the following graph. The red dot
continues to represent the formation of the Sun, and the green dot
the present day:

The
decline is striking.

By
integrating this curve forward, we can get an idea of where the Sun
lays in the final star-order of the universe. I have normalized the
graph to the (finite) number of stars that will ever exist when you
integrate out to infinity:

In
short, we find that the
Sun was born when ~79% of stars that will ever exist already existed,
and at the present moment ~90% of all stars that will ever exist
already exist.
Thus, the sun is a relative but by no means extreme latecomer to the
universe, and despite existing near the beginning of an apparently
open-ended universe its time of formation is not terribly special.

The
universe is full to bursting with hydrogen and helium, only the
tiniest fraction having been converted into heavy elements by being
consumed in stars. Naively one might assume that all this gas would
eventually condense down into stars one day. Recent results in
astrophysics are suggesting reasons, however, that this probably
won't happen – reasons for my continued use of the exponential
decay of the universal star formation rate in my above analysis, as
processes happening today continue. In my last post, I mentioned the
life histories of various types of galaxies and how they suggested
that star formation might be closer to finishing than starting. I
feel compelled to go into a little bit more detail here, speaking
more in terms of what happens to all the gas that could form stars
but doesn't using recent astrophysical results.

When
you look out into the universe, the vast vast majority of elliptical
type galaxies are very red due to the age of their stars and are not
forming stars, whether they have an internal reservoir of gas within
their dark matter halo or not – see
http://www.dailygalaxy.com/my_weblog/2014/02/giant-elliptical-galaxies-why-are-they-red-and-dead.html
for a discussion on this. Big spirals are mostly
forming stars at a steady clip, with only a few tapering down and
turning 'green' or eventually 'red' from their initial 'blue' status.
Recently, a project called GalaxyZoo which has automated and
crowdsourced the analysis of huge numbers of new galaxies observed in
the Sloan Digital Sky Survey has taken a very quantitative look at
star formation across galaxy types in the universe, and come up with
some striking conclusions:

These
studies were able to get more information than the instantaneous rate
of star formation, and look back along the history of the galaxies by
looking at light of different frequencies – huge stars that don’t
live long make lots of ultraviolet, stars like our Sun peak in the
green light, while long lived stars peak in the red. They were able
to see that among elliptical galaxies, the tiny fraction that are
star-forming mostly show evidence of recently being involved in
mergers, and that all those that are red and green colored show
spectral patterns indicative of very rapid shutdown of star
formation, faster than can be accounted for by star formation eating
up available gas. They call this fast star-formation shutdown
'quenching'. Something about their formation, either primordially or
via mergers of spirals, puts their gas into forms that cannot form
stars. The prime suspect is the initiation of regular energetic
outbursts from their large central black holes, heating the gas and
rendering it too turbulent.

This
actually dovetails interestingly with another problem in
astrophysics: the 'cooling paradox'. As I mentioned, about 90% of the
baryonic mass of the universe is in the form of X-ray hot gas clouds
blanketing entire galaxy clusters (largely outside the dark matter
halos of individual galaxies). This gas is ridiculously thin and
immensely hot, and radiating energy rapidly in the X-rays. It turns
out that when you figure out how much mass is in these gas clouds and
how much energy they are radiating in the X-rays, they should cool
and sink down to the centers of the clusters on a timescale of
gigayears, probably turning into cool gas flows onto the large
galaxies at the centers of these clusters. But they don't. Looking
back in time across the universe they are at more or less the same
temperature now as they always have been and never seem to cool
despite the fact that they are radiating energy. In recent years, for
various reasons (images of turbulence in the gas, calculations of the
available energy) the prime suspect for the energy source keeping
these gas clouds energized has become supermassive black hole jets.

Anyways,
as for spiral galaxies, they were able to model the distribution they
saw (most of which are forming stars at a steady rate, some of which
are tapering off, and some of which are red and dead) as a mixture of
populations. One population is forming stars at a steady slowly
decreasing rate, much like ours. Another is quenching on a much
slower timescale than ellipticals, indicative of a cut-off of gas
inflow into their star-forming discs and star formation then slowly
depleting their reservoir of gas over a 1-2 gigayear timescale,
likely caused by events in their immediate galactic neighborhood
disrupting the inflows of cool gas within their dark matter halos
onto their star forming discs.

All
of this suggests to me that the decline in star formation rate seen
across the cosmological past represents larger and larger fractions
of galaxies quenching along with slow decreases in star formation
rates within individual galaxies, and that the numbers I produced
above have at least some semblance of validity. I can't, of
course, rule out the possibility that galaxy quenching is a temporary
or cyclical phenomenon on very long timescales, or that there's
some special subset of galaxies that will never quench and will use
up all their gas. But these numbers are a good start.

****

These
numbers are, however, numbers about STARS and the Sun's position in
star-order. And as vital as stars are for life, we don't live
on a star. We live on a terrestrial planet. And this
makes a difference.

The
huge bolus of star formation early in the universe consisted of many
low-metallicity stars at the starts of spiral galaxies and the fast
star formation bursts of elliptical galaxies. Many of these
stars are probably not suitable for the creation of terrestrial
planets and thus biospheres-as-we-know-them. In order to get a
handle on Earth's position in planet-order we need to normalize
this. I am utterly unprepared to do this rigorously since the
astronomical community as a whole hasn't got a good handle on planet
formation - if there's one incontrovertible takeaway from the Kepler
mission, this is it. I can, however, pull up some numbers
that are better than nothing and layer them on top of my star
formation numbers and see what comes out.

I
will be taking data from a paper (Behroozi & Peeples, 2015:
http://arxiv.org/pdf/1508.01202v1.pdf)
that made the rounds last year suggesting that Earth came in the 8th
percentile of terrestrial planets (that is, 92% come after us).
Their conclusion is suspect because it comes from the assumption that
ALL gas within galactic dark matter halos will eventually form
stars. However, the authors include a nice set of metallicity
normalizations that they apply to the early universe that I will take
in its entirety, as it is far too complicated for someone like me who
doesn't study cosmology professionally to critique.

In
this work, all stars are assumed to form terrestrial planets with a
power-law dependence on metallicity and a sharp metallicity cutoff
for 'gas planet' formation is assumed. I will use both these
functions, on the chance that either of them is relevant - again,
nobody really
understands planet
formation. I find myself suspecting that the latter is more
relevant due to some talks I've seen on solar system structure and
planet formation modeling, but I really don't know.

This
work finds that using a power law metallicity dependence, the Eartth
is younger than ~83% of currently existing planets, and that with a
sharp metallicity cutoff our planetary system is younger than ~64% of
currently existing systems (both of these numbers shamelessly scraped
from graphs). Now I apply an almost certainly oversimplified
and wrong assumption: that the fraction of star systems that
have been planet-forming has been constant since the formation of the
Sun, on the theory that the pollution of heavy elements is more or
less complete.

Applying these numbers, I estimate the
following positions of the Earth and by extension our biosphere in
planet-order:

Under the power-law metallicity assumption,
Earth shows up as
younger than 72% of planetary systems that will exist.

Under
a sharp metallicity-cutoff assumption, Earth
shows up as younger than 51% of planetary systems that will exist.

In
the words of the first person I showed these calculations to, these
numbers are very interesting in that they are very boring. We
find ourselves in an unremarkable position in terms of
planet-formation order. We are not early and other explanations for
the so-called Fermi pardox must be invoked.

Wow
this post has gotten long. I still have some thoughts on other
large-scale considerations about our position in space and time -
namely why we find ourselves around a star as large and relatively
short-lived as the Sun (larger than most stars) and not, say, 30
billion years into the lifespan of a small star born at the same time
as the Sun but with a 40 billion year lifetime. I will save
that for another revisit to this topic. Stay tuned for my
original intent, talking about our own solar system.

Sunday, July 26, 2015

Those of an anthropic bent have often made much of the fact
that we are only 13.7 billion years into what is apparently an open-ended
universe that will expand at an accelerating rate forever.The era of the stars will last a trillion
years; why do we find ourselves at this early date if we assume we are a ‘typical’
example of an intelligent observer?In
particular, this has lent support to lines of argument that perhaps the answer
to the ‘great silence’ and lack of astronomical evidence for intelligence or
its products in the universe is that we are simply the first.This notion requires, however, that we are actually early in the universe when
it comes to the origin of biospheres and by extension intelligent systems.It has become clear recently that this is not
the case.

The clearest research I can find illustrating this is the
work of Sobral et al, illustrated here http://arxiv.org/abs/1202.3436
via a paper on arxivand here http://www.sciencedaily.com/releases/2012/11/121106114141.htm
via a summary article.To simplify what
was done, these scientists performed a survey of a large fraction of the sky
looking for the emission lines put out by emission nebulae, clouds of gas which
glow like neon lights excited by the ultraviolet light of huge, short-lived
stars.The amount of line emission from
a galaxy is thus a rough proxy for the rate of star formation – the greater the
rate of star formation, the larger the number of large stars exciting
interstellar gas into emission nebulae.The
authors use redshift of the known hydrogen emission lines to determine the
distance to each instance of emission, and performed corrections to deal with
the known expansion rate of the universe.The results were striking.Per
unit mass of the universe, the current rate of star formation is less than 1/30
of the peak rate they measured 11 gigayears ago.It has been constantly declining over the
history of the universe at a precipitous rate.Indeed, their preferred model to which they fit the trend converges
towards a finite quantity of stars formed as you integrate total star formation
into the future to infinity, with the total number of stars that will ever be
born only being 5% larger than the number of stars that have been born at this
time.

In summary, 95% of all
stars that will ever exist, already exist.The smallest longest-lived stars will shine for a trillion years, but
for most of their history almost no new stars will have formed.

At first this seems to reverse the initial conclusion that
we came early, suggesting we are instead latecomers.This is not true, however, when you consider where
and when stars of different types can form and the fact that different galaxies
have very different histories.Most
galaxies formed via gravitational collapse from cool gas clouds and smaller
precursor galaxies quite a long time ago, with a wide variety of
properties.Dwarf galaxies have low
masses, and their early bursts of star formation lead to energetic stars with
strong stellar winds and lots of ultraviolet light which eventually go
supernova.Their energetic lives and
even more energetic deaths appear to usually blast star-forming gases out of
their galaxies’ weak gravity or render it too hot to re-collapse into new
star-forming regions, quashing their star formation early.Giant elliptical galaxies, containing many
trillions of stars apiece and dominating the cores of galactic clusters, have
ample gravity but form with nearly no angular momentum.As such, most of their cool gas falls straight
into their centers, producing an enormous burst of low-heavy-element star
formation that uses most of the gas.The
remaining gas is again either blasted into intergalactic space or rendered too
hot to recollapse and accrete by a combination of the action of energetic young
stars and the infall of gas onto the central black hole producing incredibly
energetic outbursts.(It should be
noted that a full 90% of the non-dark-matter mass of the universe appears to be
in the form of very thin X-ray-hot plasma clouds surrounding large galaxy clusters,
unlikely to condense to the point of star formation via understood
processes.)Thus, most dwarf galaxies
and giant elliptical galaxies contributed to the early star formation of the
universe but are producing few or no stars today, have very low levels of heavy
element rich stars, and are unlikely to make many more going into the future.

Spiral galaxies are different.Their distinguishing feature is the way they
accreted – namely with a large amount of angular momentum.This allows large amounts of their cool gas
to remain spread out away from their centers.This moderates the rate of star formation, preventing the huge pulses of
star formation and black hole activation that exhausts star-forming gas and
prevents gas inflow in giant ellipticals.At the same time, their greater mass than dwarf galaxies ensures that
the modest rate of star formation they do undergo does not blast nearly as much
matter out of their gravitational pull.Some does leave over time, and their rate of inflow of fresh cool gas
does apparently decrease over time – there are spiral galaxies that do seem to have
shut down star formation.But on the
whole a spiral is a place that maintains a modest rate of star formation for
gigayears, while heavy elements get more and more enriched over time.These galaxies thus dominate the star
production in the later eras of the universe, and dominate the population of
stars produced with large amounts of heavy elements needed to produce planets
like ours.They do settle down slowly
over time, and eventually all spirals will either run out of gas or merge with
each other to form giant ellipticals, but for a long time they remain a class
apart.

Considering this, we’re just about where we would expect a
planet like ours (and thus a biosphere-as-we-know-it) to exist in space and on
a coarse scale in time.Let’s look
closer at our galaxy now.Our galaxy is
generally agreed to be about 12 billion years old based on the ages of globular
clusters, with a few interloper stars here and there that are older and
would’ve come from an era before the galaxy was one coherent object.It will continue forming stars for about
another 5 gigayears, at which point it will undergo a merger with the Andromeda
galaxy, the nearest large spiral galaxy.This merger will most likely put an end to star formation in the
combined resultant galaxy, which will probably wind up as a large elliptical
after one final exuberant starburst.Our
solar system formed about 4.5 gigayears ago, putting its formation pretty much
halfway along the productive lifetime of the galaxy (and probably something
like 2/3 of the way along its complement of stars produced, since spirals DO
settle down with age, though more of its later stars will be metal-rich).

On a stellar and planetary scale, we once again find
ourselves where and when we would expect your average complex biosphere to
be.Large stars die fast – star
brightness goes up with the 3.5th power of star mass, and thus star
lifetime goes down with the 2.5th power of mass.A 2 solar mass star would be 11 times as bright
as the sun and only live about 2 billion years – a time along the evolution of
life on Earth before photosynthesis had managed to oxygenate the air and in
which the majority of life on earth (but not all – see an upcoming post) could
be described as “algae”.Furthermore,
although smaller stars are much more common than larger stars (the Sun is
actually larger than over 80% of stars in the universe) stars smaller than
about 0.5 solar masses (and thus 0.08 solar luminosities) are usually ‘flare
stars’ – possessing very strong convoluted magnetic fields and periodically
putting out flares and X-ray bursts that would frequently strip away the ozone
and possibly even the atmosphere of an earthlike planet.

All stars also slowly brighten as they age – the sun is
currently about 30% brighter than it was when it formed, and it will wind up
about twice as bright as its initial value just before it becomes a red
giant.Depending on whose models of
climate sensitivity you use, the Earth’s biosphere probably has somewhere
between 250 million years and 2 billion years before the oceans boil and we
become a second Venus.Thus, we find
ourselves in the latter third-to-twentieth of the history of Earth’s biosphere
(consistent with complex life taking time to evolve).

Together, all this puts our solar system – and by extension
our biosphere – pretty much right where we would expect to find it in space,
and right in the middle of where one would expect to find it in time.Once again, as observers we are not
special.We do not find ourselves in the
unexpectedly early universe, ruling out one explanation for the Fermi paradox
sometimes put forward – that we do not see evidence for intelligence in the
universe because we simply find ourselves as the first intelligent system to
evolve.This would be tenable if there
was reason to think that we were right at the beginning of the time in which
star systems in stable galaxies with lots of heavy elements could have birthed
complex biospheres.Instead we are
utterly average, implying that the lack of obvious intelligence in the universe
must be resolved either via the genesis of intelligent systems being
exceedingly rare or intelligent systems simply not spreading through the
universe or becoming astronomically visible for one reason or another.

In my next post, I will look at the history of life on
Earth, the distinction between simple and complex biospheres, and the evidence
for or against other biospheres elsewhere in our own solar system.